Various properties of gypsum are beneficially desirable for making certain building and plaster products, such as gypsum fiberboard. Gypsum is a plentiful and generally inexpensive raw material which, through a process of dehydration and rehydration, can be cast, molded, or otherwise formed into useful shapes. Gypsum is further noncombustible and relatively stable when exposed to moisture.
Typically, gypsum fiberboard is prepared from slurries of alpha calcium sulfate hemihydrate (also known herein as, calcined alpha hemihydrate stucco, calcined gypsum, alpha hemihydrate stucco, hemihydrate gypsum, or calcined stucco) and cellulose fiber with an excess of water. For manufacturing gypsum fiberboard, the slurry is deposited onto a moving wire to remove water to form a mat and the formed mat is pressed to form to achieve the desired thickness and surface smoothness. During the manufacturing process, the calcined gypsum rehydrates such that the formed and pressed mat hardens. Such gypsum fiberboard is used largely for roofing board, interior wall, partition wall, and ceiling applications.
According to a process described in U.S. Pat. No. 5,320,677 to Baig, which is incorporated herein by reference in its entirety, a composite gypsum/wood fiber (GWF) material product and a process for forming the product is disclosed. In Baig, a dilute slurry of gypsum particles and cellulosic fibers are heated under pressure to convert the raw gypsum to calcium sulfate hemihydrate by calcining the gypsum in the presence of wood fibers. The dissolved calcium sulfate wets the voids in the fiber and the resulting hemihydrate eventually forms crystals in situ in the voids of the cellulose fiber. The process of Baig describes a single-stage process of producing the GWF material single stage calcination of both the gypsum particles and the cellulosic fibers. To form GWF wallboard, the calcined slurry is substantially dewatered before rehydrating the alpha hemihydrate stucco back to gypsum. Baig's single-stage process involves calcination of both gypsum and cellulosic fiber simultaneously.
U.S. Pat. No. 8,529,863 to Yokoyama et al. discloses a process for the continuous modification of dihydrate gypsum. Yokoyama discloses a step of calcining dihydrate gypsum into hemihydrate gypsum and a recrystallization step of hydrating and recrystallizing the hemihydrate gypsum in an aqueous slurry to convert it into modified dihydrate gypsum of a different crystalline form. The temperature during recrystallization is between, but not including 80° C. and 90° C. Yokoyama discloses both dry calcination and wet calcination, and a multistage processing of each step using multiple tanks. Yokoyama thus discloses a two-stage process in which the first stage converts gypsum dihydrate into hemihydrate gypsum, and the second stage converts hemihydrate gypsum to dihydrate gypsum. Yokoyama does not disclose a two-stage process in which both stages are characterized by the conversion of dihydrate gypsum into hemihydrate gypsum.
U.S. Pat. No. 7,815,889 to Luan et al., which is incorporated herein by reference in its entirety, discloses a method for calcining gypsum in a pressurized reactor by injecting combustion gasses and air into the reactor to create a fluidized bed of gypsum. The fluidized bed is heated to form calcined hemihydrate. Luan distinguishes its continuous process for calcining lower water demand hemihydrate from a batch process. In an example, gypsum is injected into a pressurized reactor; heated air, steam, and a portion of a combustion gas are injected to create a fluidized bed of gypsum; and the reactor is heated and maintained at a temperature of about 121° C. to about 149° C. (or about 250° F. to about 300° F.) and pressurized to a vapor pressure of 1.01×105 to 3.85×105 Pascal (Pa) (or from 1.0 to 3.8 atmospheres (atm)). Accordingly, the Luan method does not disclose a two-stage calcination process.
U.S. Pat. No. 3,236,509 to Blair discloses a dry calcination, single chamber process for continuous calcining of powdered gypsum rock. Powdered gypsum is fed into a chamber for calcination, followed by addition of new powdered gypsum feed at a temperature of 280° F. to 340° F. (138 to 171° C.). Blair thus discloses a dry calcination process, not a wet calcination process, and at identical temperature ranges throughout the process.
U.S. Pat. No. 3,579,599 to Anderson et al. discloses a continuous process for the production of calcium sulfate alpha-hemihydrate from gypsum. Anderson describes continuously passing a slurry of gypsum and water at super-atmospheric pressure between about 4 and 10 atm into and through a reactor, maintaining the slurry in the reactor, and injecting steam at a pressure above the reactor pressure and a temperature greater than 100° C. to convert the gypsum into calcium sulphate alpha-hemihydrate. The method controls the steam injection by providing at least one hot zone of a temperature of at least 10° C. hotter than the mean slurry temperature to control crystal nucleation. Accordingly, the Anderson continuous process is performed as a single-stage process, and not a two-stage process.
EP 2418184 to Aschern discloses mixing gypsum-containing waste materials, flue-gas desulfurization gypsum, and water to obtain an aqueous suspension, which is heated to react the components and separate alpha calcium sulfate hemihydrate. The heating temperature is in the range of 105° C. to 150° C. Accordingly, Aschern discloses a single-phase process, not a two-stage process, at a single temperature range.
U.S. Pat. No. 3,437,330 to Worner discloses a process of continuous production of alpha plaster using a wet gypsum slurry feed to produce a dry, high strength plaster by regulation of temperature and pressure. In an example, a slurry of gypsum particulate and water is fed at a controlled rate into a pressurized chamber with steam and water at a temperature between 110° C. and 125° C. and maintained therein to convert the gypsum to fine crystals of alpha hemihydrate. The reacted slurry is transferred to another chamber at a higher pressure and temperature not to exceed 180° C. and maintained therein to grow the alpha hemihydrate crystals. Thus, Worner discloses a two-stage process, where the second stage is characterized by a temperature and pressure greater than the first stage.
U.S. Pat. No. 3,337,298 to Rüter et al. discloses a process for producing alpha calcium sulfate semi-hydrate from synthetic gypsum. The process can be performed in an autoclave in which an aqueous suspension of dehydrated calcium sulfate and crystallization agents are treated at temperatures between 105° C. and 140° C., with or without pressure, to influence crystallization. Salt solutions can be used to influence crystallization at a temperature close to the boiling point of the solution, such as in the range of 90° C. to 110° C. The pH of the aqueous suspension is maintained between 1 and 5. GB 992468 and GB 1243092 to the same applicant disclose similar processes. In GB 992468, after seed crystals of calcium sulphate α-hemihydrate are formed, an aqueous suspension of synthetic calcium sulphate dihydrate is added continuously or intermittently at the temperature and pH ranges of Rüter to convert the calcium sulphate dihydrate to crystalline calcium sulphate α-hemihydrate. In GB 1243092, crystal size and growth of calcium sulphate α-hemihydrate from calcium sulphate dihydrate is regulated by the continuous or intermittent addition of seed crystals of α-hemihydrate or β-hemihydrate at the temperature range of Rüter and a pH range of 1.1 to 5. Accordingly, pH ranges above 5 are not disclosed, an identical temperature range is applicable across all processing steps, and crystalline influencing agents are required.
EP 672634 to Brosig et al. discloses production of α-CaSO4 semihydrate by continuous addition of a fine CaSO4 dihydrate and water suspension into a stirred autoclave subjected to vapour pressure. Brosig discloses partial conversion of α-CaSO4 semihydrate occurs by the pressurized wet process and residual conversion is completed in a second dry process by an autoclave subjected to vapour pressure and at temperatures exceeding 160° C. Thus, this is a single stage calcination process and no additional dihydrate is added to the second dry process for residual conversion. In addition, the second drying stage requires a drying temperature of at least 125° C., higher than the 100° C. temperature required in the present disclosure.
WO 1990/011256 to Lynn et al., incorporated herein in its entirety, discloses a process and apparatus for producing large diameter, high aspect ratio calcium sulfate microfibers. The process passes a steam heated dilute aqueous slurry having about 0.5% to 15% gypsum by weight through a pressure pump at a temperature of about 285° F. (141° C.). Laminar flow is established and the gypsum is converted to calcium sulfate alpha hemihydrate nucleates forming needle-like seed crystals. Thereafter, the crystals and unreacted gypsum is passed to a higher volume reactor and agitated, promoting radial and axial growth. Accordingly, Lynn discloses a two-stage calcination process, but with minimal gypsum loading (i.e., about 0.5-15 weight %) and at a set temperature throughout the process.
It will be appreciated that this background description has been created by the inventors to aid the reader and is not to be taken as an indication that any of the indicated problems were themselves appreciated in the art. While the described principles can, in some aspects and examples, alleviate the problems inherent in other systems, it will be appreciated that the scope of the protected innovation is defined by the attached claims and not by the ability of any disclosed feature to solve any specific problem noted herein. Thus, there is a continuing need for new and improved set gypsum-containing products and compositions used in preparing the products, particularly set accelerators, as well as methods for producing them, that solve, avoid, or minimize a problem noted above, and/or improves properties of the products.